In wind turbines the wind energy may be transferred into electrical power by ring generators. The ring generators comprise a rotor and a stator in which a rotor shaft is supported pivotably. To the rotor shaft the rotor blades of the wind turbine are attached, so that the wind energy rotates the rotor shaft and thus generates electrical power in the stator by electromagnetic induction.
In conventional wind turbines, the rotational speed of the rotor blade and thus of the rotor shaft is very slow. Thus, in conventional wind turbines gearboxes may be provided in order to provide a higher rotational speed of the rotor shaft inside the stator for providing an efficient power generation.
On the other side, the use of a gearbox may lead to a loss of efficiency due to a resistance of the gearbox. Thus, in modern wind turbines, the diameter of the stator is formed very large, so that the contact area between the inductive elements of the rotor and the coils of the stator may be spaced from the rotary axis of the rotor shaft. Thus, even when the rotor shaft rotates slowly, the relative velocity in the contact area between the rotor and the stator is increased. This leads in wind turbines to large diameters of the stators of the ring generators that have to be transported from the place of production to the place of installation.